Off-the-road tire temperature and pressure monitoring system

ABSTRACT

A tire pressure and temperature measurement system adapts a Y-block ( 30 ) to an existing valve stem ( 20 ). One branch ( 44 ) of the Y-block accepts a conventional valve core ( 42 ) and another branch ( 38 ) accepts a flexible tubular temperature sensor ( 36 ) that leads “down the throat” of the existing valve stem and into the interior of a tire ( 14, 18 ). A second embodiment is adapted for use on inside dual tires ( 18 ). Making a measurement entails attaching a pressure sensor ( 40 ) to the valve stem and attaching a hand-held processor ( 112 ) to the sensors. The hand-held processor reads the current tire pressure and temperature; executes a program (Eqs. 1-6) that accounts for measured and target temperatures, vapor pressures, and gas compressibility; aid indicates how much pressure to add or subtract to the tire to achieve an accurate final tire operating pressure. In third and fourth embodiments, respective spherical ( 70 ) and U-shaped channel ( 120 ) protective housings enclosing pressure and temperature sensors ( 72 ), a controller  76,  and a data transceiver ( 74 ) are loosely placed or attached by magnets ( 122 ) within the interior of a tire ( 126 ). A remote measurement system ( 80 ) receives at another data transceiver ( 108 ) pressure and temperature data transmitted from inside the tire while the vehicle is moving and conveys the data to a processor ( 112 ) for executing the above-described program.

RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 09/038,761, filed Mar. 11, 1998 for OFF-THE-ROAD TIRETEMPERATURE AND PRESSURE MONITORING SYSTEM, now U.S. Pat. No. 6,025,777.

TECHNICAL FIELD

This invention relates to pressure and temperature measurements and moreparticularly to systems for measuring giant off-the-road tiretemperatures and pressures under first sets of conditions and predictinginflation pressure changes required to achieve targeted operatingpressures under second sets of conditions.

BACKGROUND OF THE INVENTION

Giant off-the-road tires, particularly those employed by earth movingand mining ore vehicles, are subjected to very heavy loads that makethem susceptible to premature failure unless proper inflation pressuresare maintained. For example, FIG. 1 shows a mining ore truck 10, whichcan weigh up to 550 tons when loaded and carries its heavy loads on onlytwo axles. A front axle 12 has two single tires 14 mounted thereon and arear axle 16 has two dual tires 18 (only one “dual” shown) mountedthereon, resulting in loads of about 50 tons per tire. A typical tire isinflated through a conventional valve stem 20 (only two shown) to anoperating pressure ranging from about 85 to 185 pounds per square inchof gauge pressure (“psig”) and, when operating, may have an operatingtemperature ranging from about 100 to 255 degrees Fahrenheit (“° F.”).If the tire pressure is too high, a worst case failure mode (explosion)may occur. However, if the tire pressure is too low, the excess heatgenerated may cause separation of some of the 18 to 22 tire layers afteronly 300 hours of operation, whereas such tires normally have at least a1,000 hour operating life. Giant off-the-road tires cost about $25,000to $30,000 each, and vehicle downtime costs at least about $500 perhour. Clearly, maintaining proper tire operating pressure is an economicimperative.

Unfortunately, conventional tire pressure measurement methods requireallowing the tires to cool for about eight hours before an accurate tirepressure measurement and inflation pressure adjustment can be made. Theresulting costly downtime often leads to infrequent tire pressuremeasurements. To make matters worse, air compressors often add watervapor and compression heating of the inflation air, causing errorsbetween the measured and operating air pressures in the tires. Moreover,off-the-road vehicle operators often add fluids to tires to inhibit rimrust and scale that otherwise makes tire removal difficult. Also, newtires are stored outdoors where they can collect rainwater, some ofwhich inevitably remains in the tire when it is installed on a vehicle.Unfortunately, such fluids have vapor pressures that contribute topressure measurement errors. Tires operating under these conditions arereferred to as “wet” tires. Finally, many off-the-road vehicles operatein cold climates but are maintained in heated facilities, furthercomplicating the tire inflation pressure problem. Unfortunately,conventional gas law equations do not provide solutions to theseproblems.

There are previously known apparatus and methods for solving some of theabove-described problems. In particular, U.S. Pat. No. 5,452,608 forPRESSURE AND TEMPERATURE MONITORING VEHICLE TIRE PROBE WITH RIM ANCHORMOUNTING describes a tire rim mounted sensor probe and conductorterminal apparatus for sensing the air pressure and temperature inside atire. When the vehicle stops, a conventional electronic measuring deviceis electrically connected to the conductor terminal to convert thesensor probe data into pressure and temperature measurements.Unfortunately, the sensor probe is separately mounted through a hole inthe rim, a disadvantage that weakens the rim and requires a relativelyexpensive field retrofit to every rim. If a new rim is required, itsimilarly has to be retrofitted, adding to the downtime of vehicle. Ofcourse, a tire cool down period may be required to obtain usablereadings.

U.S. Pat. No. 5,335,540 for TIRE MONITORING APPARATUS AND METHODdescribes a tire pressure and temperature sensing apparatus that employsradio telemetry to continuously monitor tire pressure and temperaturewhile the vehicle is operating. However, as in the above-describedpatent, the pressure and temperature sensing probe is separately mountedto the rim, which has many of the above-described disadvantages.Moreover, every vehicle carries a telemeter receiver by which thevehicle driver monitors the tire operating pressures and temperatures.

There are many other patents describing tire pressure and temperaturemonitoring apparatuses and methods. Some describe sensors embedded inthe tires, others describe wheel hub mounted slip rings for conductingsensor data to a vehicle data processor, and still others describecomplex systems for inflating and deflating tires while the vehicle isoperating. However, there is no known prior method or apparatus thataccounts for pressure measurement errors caused by air compressors,water vapor pressure, and temperature changes.

What is needed, therefore, is an accurate tire pressure measuring systemthat does not require a cool down time, does not require expensiveretrofitting or weakening of tire rims, accounts for sources ofmeasurement errors, is readily transferred among tires and vehicles, andis usable on a wide variety of tire and wheel combinations.

SUMMARY OF THE INVENTION

An object of this invention is, therefore, to provide an apparatus and amethod for measuring tire pressure and predicting inflation pressurechanges that account for many sources of measurement errors withoutrequiring a tire cool down period.

Another object of this invention is to provide an apparatus and a methodfor measuring tire pressure and temperature without resorting toexpensive retrofitting or weakening of tire rims.

A further object of this invention is to provide a tire pressure andtemperature measuring apparatus and a method that is readilytransferable among tires and vehicles and is usable with a wide varietyof tire and rim combinations.

A first embodiment of a tire pressure and temperature measurement systemof this invention adapts a Y-block to an existing valve stem. One branchof the Y-block accepts a conventional valve core and the other branch ofthe Y-block accepts a flexible tubular temperature sensor that leads“down the throat” of the existing valve stem and into the interior ofthe tire. Making a measurement entails stopping the vehicle and, withouta cool down period, attaching a pressure sensor to the conventionalvalve stem and attaching a hand-held processor to the pressure andtemperature sensors. The hand-held processor reads the current tirepressure and temperature and executes a program that accounts formeasured and target temperatures, vapor pressures, and gascompressibility and indicates how much pressure to add or subtract tothe tire to achieve an accurate final tire operating pressure.

A second embodiment of the tire pressure and temperature measurementsystem employs a modified Y-block that adapts to an existing rim, angleadaptor, and extension hose of an inside dual tire. The modified Y-blockreceives an elongated temperature sensor with sufficient flexibility forfeeding down the throat of the extension hose, bending around the cornerof the angle adaptor, and protruding into the interior of the tire.Making a measurement is carried out as in the first embodiment.

A third embodiment of the tire pressure and temperature measurementsystem encloses pressure and temperature sensors and a battery poweredtelemetry transmitter inside a generally oblate protective housing, allof which is simply placed loosely within the interior of a tire when itis mounted to a rim. The telemetry transmitter battery capacity issufficient for one year of continuous operation, which exceeds theexpected tire life. In this embodiment, a telemetry receiver receivespressure and temperature data transmitted from inside the tire andconveys the data to a processor that executes the program described inthe first embodiment. The telemetry receiver can be tuned to receivedata from multiple tires while the vehicle is moving. Alternatively, thetelemetry receiver can be adapted to receive data only from a closelyadjacent tire.

A fourth embodiment of the tire pressure and temperature measurementsystem encloses the pressure and temperature sensors and the batterypowered telemetry transmitter inside a protective U-shaped channelhousing that is magnetically attached to the rim inside the tire. As inthe third embodiment, the telemetry receiver receives pressure andtemperature data transmitted from inside the tire and conveys the datato a processor that executes the program described in the firstembodiment. The telemetry receiver can be tuned to receive data frommultiple tires while the vehicle is moving. Alternatively, the telemetryreceiver can be adapted to receive data only from a closely adjacenttire.

Additional objects and advantages of this invention will be apparentfrom the following detailed description of preferred embodiments thereofthat proceed with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric pictorial view of an exemplary prior art oretruck suitable for employing this invention.

FIG. 2 is a sectional side elevation view of a first embodiment of aY-block of this invention adapted to an existing rim and valve stem.

FIG. 3 is a sectional elevation view of a second embodiment of a Y-blockof this invention adapted to an existing inside dual rim, valve stem,and extension hose.

FIG. 4 is a sectional elevation view of a third embodiment of thisinvention showing pressure and temperature sensors and a battery poweredtelemetry transmitter enclosed within a spherical protective housing.

FIG. 5 is an electrical block diagram of a tire pressure and temperaturemeasuring system based on the third and fourth embodiments of thisinvention.

FIG. 6 is an isometric cutaway view of a fourth embodiment of thisinvention showing the pressure and temperature sensors and batterypowered telemetry transmitter of FIG. 4 enclosed in a protectiveU-shaped channel housing that is magnetically attached to a tire rim.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 2 shows a first embodiment of this invention in which a Y-block 30is threaded onto existing valve stem 20, which is bolted in place on awheel rim 32. Single tire 14 is mounted to wheel rim 32, which isattached to an axle hub 34 by multiple studs and nuts (not shown). Theinterior temperature of tire 18 is sensed by a temperature sensor 36,such as a thermocouple or thermistor that is inserted through a firstbranch 38 of Y-block 30, through existing valve stem 20, and into theinterior of tire 14. A seal nut 39 that is threaded into first branch 38secures temperature sensor 36 therein and prevents air from leaking outfirst branch 38.

Tire pressure is measured by attaching a pressure sensor 40 to a valvecore 42 that is threaded into a second branch 44 of Y-block 30. Pressuresensor 40 is preferably an ECLIPSE model 9251702, manufactured by DataInstruments, Inc. of Acton, Mass. Conventional means are employed toconnect, condition, and digitize the signals generated by temperatureand pressure sensors 36 and 40. The digitized signals are processed asdescribed with reference to the EXAMPLE herein to compute how much airpressure to add to, or subtract from, tire 14.

Preferably, temperature sensor 36, pressure sensor 40, and theassociated connecting, signal conditioning, and digitizing circuitry aresecurely housed and mounted to rim 32, and a portable computer (notshown) is plugged into an interface port on the housing to compute howmuch pressure to add to, or subtract from, tire 14.

FIG. 3 shows a second embodiment in which a Y-block 50 is connected toan extension hose 52 that connects through an elbow 54 to existing valvestem 20 on inside dual tire 18. This embodiment requires an elongated,flexible temperature sensor 56 to make a 90-degree bend before enteringthe tire. Preferably, a flexible thermocouple housed in a “speedometercable-style” housing is sufficiently flexible to traverse such bendswhile surviving the harsh environments of off-the-road vehicle tires.

As in the first embodiment, Y-block 50 has first and second branches 58and 60 and is coupled to associated connecting, signal conditioning, anddigitizing circuitry that are securely housed and mounted to rim 32. Inthis embodiment, however, inside dual tire 18 is mounted to an insidedual wheel rim 62, which is bolted in place on an inside dual axle hub64.

FIG. 4 shows a third embodiment of this invention in which a generallyoblate spherical housing 70 encloses a pressure and temperature sensor72, a data transceiver 74, a controller 76, and batteries 78, all ofwhich are simply inserted loosely inside the tire when it is installedon the vehicle. Pressure and temperature, sensor 72 s preferably a modelPPT-R manufactured by Honeywell, Inc. of Plymouth, Minn. Pressure andtemperature sensor 72 is housed within a 1.375 inch diameter, 3.65 inchlong stainless steel cylinder having a data connector at one end and a ¼inch NPT female pipe threaded sensor access hole at the other end. Themodel PPT-R includes signal conditioning, digitizing, and communicatingelectronics within its package.

Housing 70 may be shaped like a ball, a football, or any other suitableshape that will not damage the interior of the tire and protects thecomponents inside housing 70. In a developmental prototype, housing 70was a 10 inch diameter polypropelene ball having about a 0.187 inch wallthickness and six 4.0 inch diameter holes evenly distributed about itssurface. Such a ball is available from pet stores as a ferret toy namedthe “Jolly Ball,” which is manufactured by Jolly Pet, Inc. of Ravenna,Ohio. The holes are of a suitable size and distribution to allowassembly of the components inside housing 70. Of course, other housingconstructions are possible including mated hemispheres, cages, bluntedcylinders, tauruses, dumb-bell shapes, and inflatable shapes. Testingrevealed that a 20 inch diameter housing is preferred to keep theantenna associated with transceiver 74 at least 10 inches off the tirefloor, thereby increasing transmission efficiency through the tire sidewall.

Sensed pressure and temperature data are telemetered by transceiver 74to a remote measurement system 80 while the vehicle is in operation.Tire life can be significantly improved by remotely measuring tirepressure and temperature at the end of each ore-transporting run,computing in remote measurement system 80 any pressure changes required,and adjusting the tire pressure accordingly before the nest run. Thecomputing is carried out as described with reference to the EXAMPLEherein.

A two inch diameter, spherically faced first end cap 82 is attached bysix screws 84 (only two shown) to the interior of housing 70. Thespherical face has a radius of curvature chosen to match the insideradius of housing 70. The center of first end cap 82 is drilled andtapped with ¼ inch NPT female threads. A ¼ inch NPT male nipple 86mechanically connects pressure and temperature sensor 72 and first endcap 82 and provides an opening between pressure and temperature sensor72 and the air inside the tire.

A second end cap 90 having a similar spherical face is attached by sixscrews 92 (only two shown) to the side of housing 70 that isdiametrically opposed to the first end cap mounting position. Second endcap 90 includes a cylindrical protrusion 94 that preferably has the samediameter as pressure and temperature sensor 72. An aluminum center tube96 having slotted ends and an inside diameter chosen to fit snugly overthe diameters of protrusion 94 and pressure and temperature sensor 72extends between and is secured thereto by clamps 98 fitted around theslotted ends. Thereby, center tube 96, pressure and temperature sensor72, and male nipple 86 form a rigid axle extending through the center ofhousing 70 with first and second end caps 82 and 90 acting as axle hubs.Pressure and temperature sensor 72, data transceiver 74, controller 76,and batteries 78 are all securely mounted to and interconnected throughcenter tube 96.

FIG. 5 shows the interconnections among electronic subassembliesimplementing a preferred tire pressure and temperature measuring system100 of this invention, which employs the third embodiment describedabove with reference to FIG. 4. Housing 70 encloses pressure andtemperature sensor 72, data transceiver 74, controller 76, a batterypack 102, and a voltage regulator 104.

Controller 76 preferably includes a model MC68HC11 CMOS microprocessorthat is manufactured by Motorola, Inc. located in Phoenix, Ariz. Themicroprocessor includes on chip random access memory and electricallyerasable programmable read-only memory to support program memory anddata transmission functions. Controller 76 intercommunicates withpressure and temperature sensor 72 through a model LT1180 differentiallogic driver/receiver (not shown), which is manufactured by LinearTechnology Corporation of Milpitas, Calif. Pressure and temperature datareceived by controller 76 are transferred along with a datasynchronization clock to data transceiver 74, which may be a MICROSTAMP20M remote intelligent communications unit model MSEML256X10SGmanufactured by MICRON Technology, Inc. located in Boise, Id. Datatransceiver 74 includes an on chip 256 bit data memory and employsdirect sequence spread spectrum transmission at a 2.44175 GHz centerfrequency. Reception employs a differential phase shift keyed modulatedbackscatter in the 2.400 to 2.4835 GHz band on a 596 kHz subcarrier. Asimple circuit board “patch” antenna 106 permits a pair of such datatransceivers and antenna to have a line of sight range of about 15meters. A range greater than about 3 meters is suitable forcommunicating with remote measurement system 80 of this invention.

Alternatively and preferably, data transceiver 74 is a conventional 900MHz data transceiver, such as ones employed in wireless telephones, witha flexible quarter wavelength vertical antenna.

The above-described circuits receive 5.5 to 6.0 volt DC power fromvoltage regulator 104, which is preferably a model LT1121 manufacturedby Linear Technology Corporation of Milpitas, Calif. Voltage regulator104 receives nominal 7.5 volt DC power from battery pack 102 thatincludes a group of six series connected batteries 78 (FIG. 4), whichare preferably “super C” sized nickel-cadmium or lithium cells that areavailable from various manufacturers. Batteries 78 are evenlydistributed around and clamped to center tube 96 near the end that isattached to second end cap 90. Because it employs on-demand burst datatransmission, tire pressure and temperature measuring system 100 has avery low standby power drain, resulting in a typical battery lifetime ofabout one year when lithium batteries are employed.

Remote measurement system 80 includes a data transceiver 108 and a patchantenna 110 that are similar to and intercommunicate with datatransceiver 74 and patch antenna 106. Data transceiver 108intercommunicates with a processor 112, such as a conventional laptoppersonal computer that includes a memory 114. Because remote measurementsystem 80 is most suitably hand-held, processor 112 and memory 114 arepreferably a model PC9000-A/D manufactured by DAP Technology Corporationof Tampa, Fla. Of course, a wide variety of PC types are usableincluding tower and desk top versions.

FIG. 6 shows a fourth embodiment of this invention in which a generallyU-shaped channel 120 encloses and provides mounting points for pressureand temperature sensor 72 (not shown), data transceiver 74, controller76 (not shown), and batteries 78 (not shown). U-shaped channel 120 hasat least one but preferably four magnets 122 attached to the terminalmargins of its side flanges for magnetically clamping U-shaped channel120 to a wheel 124 when a tire 126 is being installed on the vehicle.Magnets 122 may be selected from among various commercially availabletypes, but preferably are those made from rare-earth and ceramiccompositions and having pole pieces attached to each side that areshaped to mate to substantially flat surfaces. Field testing has shownthat such magnets have sufficient flux density to firmly secure U-shapedchannel 120 to wheel 124 under any acceleration loading conditionexperienced by the vehicle. Optimal magnetic coupling of magnets 122 towheel 124 is preferably attained by orienting the long axis of U-shapedchannel 120 substantially parallel to the rotational axis of wheel 124as shown in FIG. 6. Of course, U-shaped channel 120 is but one possibleshape for implementing the invention.

Employing magnets 122 eliminates the need for drilling mounting holes inwheel 124 and, thereby, prevents weakening of wheel 124. U-shapedchannel 120 preferably extends about 3.5 inches (8.9 cm) above wheel124, including 0.75 inches (1.9 cm) for magnets 122, and is about 4inches (10.2 cm) wide by 6 inches long (15.25 cm), which is asufficiently volume to enclose pressure and temperature sensor 72, datatransceiver 74, controller 76, and batteries 78 without touchinginterior side walls 128 of tire 126. Tire 126 is typically mountedbetween a flange 130 and a rim 132 that both extend from wheel 124.

The above-described magnetic mounting of U-shaped channel 120 to wheel124 provides a sufficient separation of a flexible vertical antenna 134from a floor 136 of tire 126 to provide adequate communicationsefficiency of data transceiver 74 through side walls 128 of tire 126.Flexible vertical antenna 134 preferably extends perpendicularly fromthe center member of U-shaped channel 120.

As in the third embodiment of this invention, sensed pressure andtemperature data are telemetered by transceiver 74 to a remotemeasurement system 80 while the vehicle is in operation.

In operation, a vehicle, such as mining ore truck 10 (FIG. 1), has atire pressure and temperature measuring system 100 installed in each oftires 14 and 18. When ore mining truck 10 approaches remote measurementsystem 80, an operator commands processor 112 to interrogate each tirepressure and temperature measuring system 100 regarding the currentpressure and temperature inside each of tires 14 and 18. Processor 112conveys the command to data transceiver 108 that transmits in sequencean interrogation burst to each of data transceivers 74, which each inturn exit standby mode, retrieve from the memory of associatedcontroller 76 the current pressure and temperature data received frompressure and temperature sensor 72, and transmit the retrieved data toremote measurement system 80.

Processor 112 stores in memory 114 the current pressure and temperaturedata associated with each of tires 14 and 18 and follows the processdescribed below and with reference to FIG. 6 to accurately predict thetire pressure reading that would result if the tires were allowed tocool down to a predetermined temperature. If the predicted pressure atthe predetermined temperature is outside an optimal inflation pressurerange, mining ore truck 10 is stopped and its tire pressures adjustedaccordingly.

The calculation process followed by processor 112 is based on equation(1), which expresses the relationships among the current and finaltemperatures, pressures, vapor pressures of water, and compressibilityfactors of a gas confined in a fixed volume.

Initial and Final Pressure Relationships: $\begin{matrix}{P_{2} = {{\frac{Z_{2}}{Z_{1}}\left( {P_{1} - P_{W1}} \right)\frac{T_{2}}{T_{1}}} + P_{W2}}} & \text{Eq.~~(1)}\end{matrix}$

where:

P₂=Final (predicted) pressure (atm), from Eq. (6)

P₁=Current (measured) pressure (atm), from Eq. (6)

T₂=Final (target) temperature in degrees Rankine (° R)*, from Eq. (7)

T₁=Current (measured) temperature (° R), from Eq. (7)

P_(W2)=Water vapor pressure (atm) at temperature T₂, from Eqs. (2) and(5)

P_(W1)=Water vapor pressure (atm) at temperature T₁, from Eqs. (2) and(5)

Z₂=Gas compressibility at pressure P₂, from Eqs. (3) or (4)

Z₁=Gas compressibility at pressure P₁, from Eqs. (3) or (4)

*The Rankine scale is the Fahrenheit-based absolute temperature scalejust as the Kelvin scale is the Centigrade-based absolute temperaturescale. Therefore, 180° R separate the freezing and steam points ofwater, whereas 100° K separate the freezing and steam points of water.

Because vapor pressures and compressibility factors Z are used tocorrect for non-ideal conditions, equation (1) does not rely onwell-known ideal gas law equations known as Boyle's law and Charles'law. The vapor pressure relationships are described with reference toequation (2) and the compressibility factor relationships are describedwith reference to equations (3) and (4), which were statisticallyderived using nitrogen and air compressibility data published in Perry'sChemical Engineers' Handbook, 4th and 6th editions.

Moreover, because the compressibility factors depend on the finalpressure and temperature, equation (1) is used twice to calculate anaccurate final pressure. Equation (1) is first used to calculate a trialfinal pressure assuming that Z₁ equals Z₂. Then, using the trial finalpressure, values for Z₁ and Z₂ are calculated. Then, equation (1) isused the second time to calculate an accurate final pressure value. Anexample calculation process is described later.

As mentioned above, equation (1) includes vapor pressure correctionsP_(W1) and P_(W2) for wet tires. Equation (2) is used to calculate thevapor pressure of water at the current and final temperatures. Vaporpressure correction of equation (1) is necessary when a tire enclosessufficient water for saturation, which occurs if any liquid water ispresent at the warmer operating temperature; that is, not all of thewater is vaporized. At the colder temperature, saturation occurs whenthe volume of liquid water is only about 0.5% of the total enclosed tirevolume.

Vapor pressure of water: $\begin{matrix}{P_{W} = e^{11.113 - \frac{6369.6}{T + 360}}} & (2)\end{matrix}$

where: P_(W) is the partial pressure in (atm) of water and T is the tireinterior air temperature in ° F.

Equation (2) was derived by employing statistical regression. Table 1shows a comparison of measured and calculated (using equation 2) vaporpressures.

TABLE 1 Temp Vapor Pressure ° F. psia Calc. % Error  20 0.050 0.052 2.58 30 0.081 0.080 −1.58  40 0.122 0.120 −1.70  50 0.178 0.176 −0.90  600.256 0.255 −0.29  70 0.363 0.363 0.15  80 0.507 0.509 0.41  90 0.6980.702 0.55 100 0.949 0.955 0.63 110 1.274 1.282 0.61 120 1.692 1.7000.47 130 2.221 2.229 0.35 140 2.887 2.890 0.12 150 3.716 3.711 −0.14 1604.739 4.718 −0.44 170 5.99  5.945 −0.76

The maximum relative error is 2.58% at 20° F., which error is less than0.002 psi (the percentage error multiplied by the vapor pressure of thewater). The 0.002 psi error is very small compared to typical tireoperating pressures over 100 psi. The maximum absolute error usingequation (2) is 0.046 psi at 170° F., which is still less than 0.05% ofthe typical tire operating pressure.

As mentioned above, equation (1) employs compressibility factors Z tocorrect the pressure, volume, and temperature relationships of realgases. Equations (3) and (4) express the compressibility factors Z forair and nitrogen respectively and were statistically obtained fromactual data, rather than using generalized charts. The resultantequations are:

Compressibility factor Z of air: $\begin{matrix}{Z = {0.99980 + {0.0025335 \times P} - \frac{1.5303 \times P}{T}}} & (3)\end{matrix}$

Compressibility factor Z of nitrogen: $\begin{matrix}{Z = {0.99979 + {0.0074727 \times P} - \frac{2.7270 \times P}{T} - {4.7453^{- 6} \times P \times T}}} & (4)\end{matrix}$

where: Z equals the compressibility factor for nitrogen or air, P equalsthe pressure in atmospheres (atm), and T equals the temperature in ° R.

The comparisons of measured and calculated data and the relative errorsresulting from using equations (3) and (4) are shown below in respectiveTables 2 and 3.

TABLE 2 Air Compressibility Temp Pressure % ° F. psig Z Z Error −10  440.9963 0.9963 −0.00 −10  58 0.9957 0.9955 0.02 −10  88 0.9935 0.9937−0.02 −10 130 0.9911 0.9912 −0.01 −10 132 0.9908 0.9911 −0.03 −10 2750.9822 0.9827 −0.05 80  44 0.9988 0.9986 0.02 80  58 0.9987 0.9983 0.0480  88 0.9980 0.9977 0.03 80 130 0.9974 0.9968 0.06 80 132 0.9972 0.99680.04 80 275 0.9950 0.9939 0.11 170  44 1.0001 1.0002 −0.01 170  581.0002 1.0003 −0.01 170  88 1.0002 1.0005 −0.03 170 130 1.0004 1.0008−0.04 170 132 1.0004 1.0008 −0.04 170 275 1.0014 1.0019 −0.05

TABLE 3 Nitrogen Compressibility Temp Pressure % ° F. psig Z Z Error −10−0 0.9992 0.9991 −0.01 −10 58 0.9960 0.9962 0.02 −10 130 0.9924 0.99270.03 −10 275 0.9857 0.9855 −0.02 80 −0 0.9998 0.9996 −0.02 80 58 0.99900.9991 0.01 80 130 0.9983 0.9984 0.01 80 275 0.9971 0.997  −0.01 170 −01.0001 0.9999 −0.02 170 58 1.0007 1.0006 −0.01 170 130 1.0011 1.00130.02 170 275 1.0029 1.0028 −0.01

Tables 2 and 3 show that comparing the calculated compressibility valuesand actual data results in suitably low errors for both gases of lessthan 0.06%.

The following equations are used to convert between commonly usedpressure and temperature units.

Pressure Unit Conversion: P(psig)=[P(atm)−1]×14.696  (5)  TemperatureUnit Conversion: T(° F.)=T(° R)−460  (7) $\begin{matrix}{{{Pressure}\quad {Unit}\quad {Conversion}\text{:}\quad P\quad ({atmosphere})} = \frac{P\quad ({psia})}{14.696}} & (6)\end{matrix}$

As described in the following example, processor 112 employs equations(1) through (7) to accurately predict a tire pressure that would resultwere the tire allowed to cool from a current temperature to a finaltemperature.

EXAMPLE

Tire pressure and temperature measuring system 100 and remotemeasurement system 80 determine that an operating wet tire has ameasured 140 psig current air pressure and a measured 125° F. currentair temperature. A system operator needs to know what the tire pressurewould be if it was allowed to cool to 40° F. The applicable steps andexample calculations are set forth below with relevant variablesunderlined.

In a first step, processor 112 computes water vapor pressure at 125° F.and 40° F. using equation (2):

P_(W1)=e{circumflex over ( )}{11.113−6369/(125+360)}=0.13279 atm

P_(W2)=e{circumflex over ( )}{11.113−6369/(40+360)}=0.00815 atm

In a second step, processor 112 converts the current pressuremeasurement P₁ psig units to atm units using equation (5):

140 psig={P₁(atm)−1}×14.696

Solving for P₁(atm) yields:

P₁(atm)=140/14.696+1=10.526 atm

In a third step, processor 112 converts ° F. units to ° R units usingequation (7):

T₁° R=125° F.+460=585° R

T₂° R=40° F.+460=500° R

In a fourth step, processor 112 sets the ratio Z₂/Z₁ equal to one andcomputes a trial pressure P₂ using equation (1):

Trial P₂=(1/1)×(10.526−0.13279)×(500/585)+0.00815=8.8916 atm

In a fifth step, processor 112 computes Z₂ and Z₁ using equation (3) andthe trial P₂. Equation (4) would be used if the system operatorspecified that the tire was filled with nitrogen rather than air:

Z=0.99980+0.0025335×10.526−1.5303/585×10.526=0.99894

Z=0.99980+0.0025335×8.8916−1.5303/500×8.8916=0.99511

In a sixth step, processor 112 computes a final P₂(atm) by recalculatingequation (1) with the calculated values of Z₂ and Z₁:

P₂=(0.99511/0.99894)(10.526−0.13279)(500/585)+0.00815=8.8575 atm

In a seventh step, processor 112 converts final P₂ psi atmosphere unitsto P₂ psi gauge units using equation (5):

P₂(psig)=(8.75−1)14.696=115.47 psig

End of EXAMPLE.

Because the temperature sensors of this invention sense the temperatureof the air contained in a tire, the same general process may also beemployed to determine temperature adjustments cause by compressionheating, expansion cooling (as the compressed air enters the tire), tirecasing temperature caused by outside storage, and hot wheel and drumresident heat.

For example, an ambient inflation adjustment (“AIA”) may be calculatedfor the original inflation of a newly installed tire or thereinstallation of a used tire. When using equation (1) to calculate theAIA, T₁ is the ambient outside air temperature (or the workingenvironment temperature), and T₂ is the temperature of the compressedair used to inflate the tire (in this example, the temperature of aheated maintenance facility). Assume a 40.00×57 tire has a coldinflation pressure specification of 110 psig at 65° F. The tire isinflated inside a 65° F. maintenance facility, whereas its operatingenvironment temperature will be 0° F. Using equations (1) through (7)reveals that the tire must be inflated to 128 psig at 65° F. if it is toachieve a 110 psig operating pressure after cooling to 0° F.

When using equation (1) as a prediction formula, T₁ is redefined as theoriginal inflation air temperature inside the tire, T₂ is the currentlymeasured air temperature inside the tire, and P₁ is the originalinflation air pressure inside the tire. Equations (1) through (7) areused to calculate P₂, the current air pressure inside the tire. P₂ isused as a baseline pressure that is compared with the currently measuredpressure reading to determine whether to increase, decrease, or maintainthe current pressure inside the tire.

Skilled workers will recognize that portions of this invention may beimplemented differently from the implementations described above for apreferred embodiment. For example, processor 112 could employ lookuptables stored in memory 114 to fetch precalculated values usable in manyof the steps set forth above. This would be particularly useful forrapidly retrieving values in the vapor pressure and unit conversionsteps. It is also conceivable that some or all of the steps could beperformed by the processor in controller 76 upon receipt of aninterrogation command that included a final temperature value.

Of course, skilled workers will understand how to use a wide variety ofprogramming languages and computer types to implement the invention.Likewise, the invention may be implemented with other than the sensors,batteries, and electronic and mechanical components described above. Forexample, the data transceivers may employ other frequencies andmodulation types or may be deleted in favor of a device that employsinductive or capacitive coupling directly through the tire. In thelatter alternative, the vehicle may need to be stopped and the deviceheld against the tire.

Also, in addition to taking on a variety of shapes and constructions,housing 70 need not be loosely enclosed within a tire, but may be bondedor otherwise attached to the tire or wheel without weakening them, orslung from the wheel inside the tire interior to increase the height ofpatch antenna 106 off the tire floor. Range may also be increased bymaking patch antenna 110 a directional antenna with gain. Of course,many different antenna designs would be effective for either transceiverdepending on the particular application. Likewise the invention can beadapted to work with many different tire shapes, sizes, and applicationsincluding automobile, aircraft, and truck tires.

It will be obvious to those having skill in the art that many changesmay be made to the details of the above-described embodiments of thisinvention without departing from the underlying principles thereof.Accordingly, it will be appreciated that this invention is alsoapplicable to pressure and temperature measuring and predictingapplications other than those found in the inflation of heavy trucktires. The scope of the present invention should, therefore, bedetermined only by the following claims.

What is claimed is:
 1. Apparatus for measuring a pressure and atemperature of gases in an interior of a tire mounted to a wheel,comprising: a housing attached to the wheel by at least one magnet andenclosed within the interior of the tire; pressure and temperaturesensors mechanically coupled to the housing and sensing a currentpressure and a current temperature of the gases in the interior of thetire; a transmitter mechanically coupled to the housing and conveying tooutside the tire data indicative of the current pressure and the currenttemperature of gases in the interior of the tire; and a receiverreceiving the data indicative of the current pressure and the currenttemperature of gases in the interior of the tire.
 2. The apparatus ofclaim 1 in which the tire is an off-the-road vehicle tire.
 3. Theapparatus of claim 1 further including a processor coupled to thereceiver, in which the processor processes the data indicative of thecurrent pressure and current temperature of gases in the interior of thetire to generate for a predetermined future temperature of the interiorof the tire an accurate prediction of a future pressure of the gases inthe interior of the tire.
 4. The apparatus of claim 3 in which thereceiver and the processor are associated with a measurement system thatis separated by a distance from the transmitter.
 5. The apparatus ofclaim 4 in which the distance is at least 3 meters.
 6. The apparatus ofclaim 1 in which the housing includes a generally U-shaped channel. 7.The apparatus of claim 6 in which the U-shaped channel includes sideflanges and in which at least one magnet is attached to each of the sideflanges.
 8. The apparatus of claim 7 in which the pressure andtemperature sensors and the transmitter are mounted in the U-shapedchannel and between the side flanges.
 9. A method for measuring apressure and a temperature of gases in an interior of a tire that ismounted to a wheel, comprising: mounting a generally U-shaped housingmagnetically to the wheel and within the interior of the tire; mountingpressure and temperature sensors to the housing; sensing a currentpressure and a current temperature of the gases in the interior of thetire; transmitting to outside the tire data indicative of the currentpressure and the current temperature of gases in the interior of thetire; and receiving the data indicative of the current pressure and thecurrent temperature of gases in the interior of the tire.
 10. The methodof claim 9 further including processing the data indicative of thecurrent pressure and current temperature of gases in the interior of thetire to generate for a predetermined future temperature of the interiorof the tire an accurate prediction of a future pressure of the gases inthe interior of the tire.
 11. The method of claim 10 in which theprocessing includes computing a water vapor pressure at the current andpredetermined future temperatures.
 12. The method of claim 10 in whichthe processing includes converting gauge pressure units to atmospherepressure units.
 13. The method of claim 10 in which the processingincludes converting Fahrenheit temperature units to Rankine temperatureunits.
 14. The method of claim 10 in which the processing includescomputing a trial pressure by employing a nonideal gas equation thataccounts for a water vapor pressure at the current and predeterminedfuture temperatures.
 15. The method of claim 14 in which the processingincludes computing for the current and the trial pressure a current anda future gas compressibility factor.
 16. The method of claim 15 in whichthe processing includes computing the future pressure of the gases inthe interior of the tire by employing the nonideal gas equation andusing the current and future gas compressibility factors as variables.17. Apparatus for measuring a pressure and a temperature of gases in aninterior of a tire mounted to a wheel, comprising: a housing attached tothe wheel by at least one magnet and enclosed within the interior of thetire; pressure and temperature sensors coupled to the housing andsensing a current pressure and a current temperature of the gases in theinterior of the tire; a transmitter coupled to the housing and conveyingto outside the tire data indicative of the current pressure and thecurrent temperature of gases in the interior of the tire; a receiverreceiving the data indicative of the current pressure and the currenttemperature of gases in the interior of the tire; and a processorcoupled to the receiver, the processor processing the data indicative ofthe current pressure and current temperature of gases in the interior ofthe tire and generating for a predetermined future temperature of theinterior of the tire an accurate prediction of a future pressure of thegases in the interior of the tire.
 18. The apparatus of claim 17 inwhich the tire is an off-the-road vehicle tire.
 19. The apparatus ofclaim 17 in which the receiver and the processor are associated with ameasurement system that is separated by a distance from the transmitter.20. The apparatus of claim 19 in which the distance is at least 3meters.
 21. The apparatus of claim 17 in which the housing includes agenerally U-shaped channel.
 22. The apparatus of claim 21 in which theU-shaped channel includes side flanges and in which at least one magnetis attached to each of the side flanges.
 23. The apparatus of claim 22in which the pressure and temperature sensors and the transmitter aremounted in the U-shaped channel and between the side flanges.